I don't want to discourage you, but if you know the subject matter and are inclined to work on a book, you should just write your own from scratch as the sole author: the normal way most books are written.

(I personally wouldn't write a book about the shell. I would be too paralyzed by the thought that at any moment, Stéphane Chazelas might crack it open.)

The amount of people writing books on subjects as a way of learning something new, is too high. I’ve seen so much trash these days. Especially game design or programming books.